Direct injection of fuel into the combustion chamber of a gasoline direct injection (GDI) internal combustion engine potentially saves fuel, reduces emissions, and increases torque/power compared with conventional port fuel injection (PFI) engines. Fuel is sprayed directly into the combustion chamber where it vaporizes and mixes with air and is later ignited by a spark plug. The main fuel saving mechanism of homogeneous charge GDI is charge cooling from the fuel vaporization process that allows a higher compression ratio for more efficient engine operation. Charge cooling is also responsible for the increased torque potential of GDI engines via higher volumetric efficiency at full load. Air inducted into the engine is denser due to charge cooling, thereby allowing more air to be inducted and more power to be produced. Reduced emissions from GDI engines are possible compared with PFI engines due to stratified charge combustion using split injection during cold start, an operating condition contributing a large fraction of overall emissions. Split injection produces lower emissions during cranking by minimizing injected fuel mass and reducing liquid fuel surface wetting. It also enables faster catalyst light-off through high-heat flux retarded spark timing.
GDI engines, however, potentially suffer from poorer fuel-air mixing compared with PFI engines. Mixing in GDI engines is controlled by the interaction of the fuel spray with the turbulent air flow in the cylinder. Therefore, mixing is dependent on time, the spatial distribution of the fuel spray, and the in-cylinder charge motion characteristics. To evenly distribute the fuel through the combustion chamber, the interaction between the spatial targeting of the fuel jet(s) and the air motion is optimized. Also, the time for mixing is maximized during the intake stroke. If fuel is injected too late during the intake stroke and/or the fuel is not sufficiently dispersed by the injector spray pattern targeting, there is insufficient time for vaporization and complete mixing. This results in lean and rich regions. Combustion of the rich zones results in incomplete combustion as indicated by elevated CO emissions and incomplete utilization of the oxygen in the charge mixture, and thus lower combustion efficiency leading to higher fuel consumption. If, on the other hand, fuel injection is initiated too early, fuel impinges on the piston creating a liquid film that may survive the mixing process and produce soot emissions. Generally speaking, a GDI engine has a soot/mixing tradeoff where undesirable soot emissions are produced when using early start of injection (SOI) and less than optimal fuel efficiency with later SOIs.
Typically, a portion of the fuel is injected during the late compression stroke to provide a rich zone in the vicinity of the spark plug to aid in cold starting. An injector spray pattern and combustion system optimized for part load mixing alone does not necessarily provide the desired rich fuel cloud in the vicinity of the spark plug at the time of ignition for stable combustion at cold-start conditions.
A challenge lies in designing a spray pattern and combustion chamber shape that produces good mixing to reduce soot emissions and provide good thermal efficiency at part load while also allowing for a stratified mixture to be formed at cold start with a rich zone at the spark plug. In some combustion systems, a deep bowl of complicated geometry is provided in the piston top to partially contain the rich fuel in a region near the spark plug to promote a rich zone for cold start robustness.